It is desirable that a stepping motor which is used for zoom drive or the like of a small camera installed in a portable device should have a small planar size and be thin. In case where it is used in a portable device, the drive source is often a battery and should desirably have a high motor efficiency and low current consumption. When control with high positional precision is desirable as in zoom drive of a camera, the rotor of a stepping motor is made to have multipoles in such a way as to guarantee a relatively high rotational position accuracy, e.g., to be able to designate the rotational position of 20 steps/rotation or so. When a rotor of a small diameter is made to have multipoles, the width of the magnetic poles of the stator that should be selectively made to face the magnetic poles of the rotor becomes smaller, saturation is likely to occur and leakage of the magnetic flux is likely to become large. Further, miniaturization and multipolarization of the rotor make the anisotropy of the shapes of individual electromagnet portions of the rotor non-negligible, and the magnetized states of the individual electromagnet portions of the rotor may become imperfect to call them a permanent magnet.
There has been a proposal to arrange members of a soft magnetic material close to the rotor in order to improve the rotational performance of such a stepping motor or the stop stability in the characteristics at the time of step rotation (Patent Literature 1, Patent Literature 2).
Patent Literature 1: Unexamined Japanese Patent Application KOKAI Publication No. 2003-32991
Patent Literature 2: Unexamined Japanese Patent Application KOKAI Publication No. 2002-136095